To examine the use of interleukin 2 as a therapeutic agent for human renal cell carcinoma the peripheral blood lymphocytes derived from patients with renal cell carcinoma were grown in interleukin 2 and tested in a 4-hour 51chromium release cytotoxicity assay against autologous cultured tumor cells. Peripheral blood lymphocytes, which were cultured separately in medium alone, mitomycin C-treated tumor cells, 20 per cent interleukin 2 and tumor cells plus interleukin 2, were used as effector cells. In all patients neither peripheral blood lymphocytes in medium alone nor in mitomycin C-treated tumor cells could lyse their own tumor cells. However, in 5 of 7 patients peripheral blood lymphocytes grown in interleukin 2 regardless of the presence of mitomycin C-treated tumor cells were capable of causing 5.7, 9.4, 12.7, 36.1 and 52.5 per cent lysis at effector/target cell ratios of 25, 50, 50, 50 and 25, respectively. However, more significant cytotoxicity was not obtained by peripheral blood lymphocytes cultured with tumor cells and interleukin 2. These results suggest that interleukin 2 might become a useful agent for patients with renal cell carcinoma.